1. Field of the Invention
The present invention relates to automatic rangefinder devices of the type having means for projecting a beam of light to illuminate a spot on a distant object, and an image sensor for sensing the apparent location of the spot to determine the distance to the object; and more particularly, to an improved light source for generating said beam of light.
2. Description of the Problems
In one type of automatic rangefinder, a beam of light is projected upon an object in a scene to form an illuminated spot on the object. An image of the scene including the illuminated spot, is formed on a linear image sensor. The location of the illuminated spot on the linear image sensor is determined to measure the distance to the object in the scene. An example of such a rangefinder device for use in a photographic camera is shown in U.S. Pat. No. 4,274,735 issued June 23, 1981 to Tamura et al.
The general arrangement of elements and the mode of operation of a rangefinder device of the type shown by Tamura et al, will be described with reference to FIG. 1. The apparatus comprises a beam-forming portion shown by way of example, as a light emitting diode (LED) 10 and a lens 12. The beam is projected along a path 14 to illuminate a spot on an object O.sub.1 in a scene. The scene is sensed by an image sensing portion comprising a second lens 16 and a linear array of photosensors 18. The signals produced by the photosensor array are analyzed by control electronics 20 to determine the position of the illuminated spot in the scene and to produce a signal representing the distance to the object. As shown by example in FIG. 1, the apparent position of the illuminated spot in the scene is a function of distance along light path 14 to the object. For an object O.sub.1 located at a distance D.sub.1 from the rangefinding device, the image of the illuminated spot will fall on the image sensor at location S.sub.1. For an object O.sub.2 at a further distance D.sub.2, the image of the spot will fall on the image sensor at a location S.sub.2. By examining the output of the image sensor, the control electronics 20 determines (for example, by comparing the output of the sensor elements to determine that output which is a maximum) the location of the illuminated spot in the scene and thereby the distance to the object.
In general, the resolution of such a rangefinder is limited by the baseline distance between the LED 10 and the image sensor 18. The ability to increase the resolution of the rangefinder by increasing the baseline distance is limited in modern compact photographic cameras.
As shown in FIG. 1, the image sensor 18 is arranged with its long axis perpendicular to the optical axis of lens 16. Since lens 16 is a very fast lens (e.g. F/1) to maximize the amount of light gathered from the scene, its depth of field will be relatively shallow and only objects at a narrow range of distance will be in focus on the image sensor 18, thereby reducing the sensitivity of the rangefinding device at other distances. Another problem encountered with the rangefinding device of the type shown in FIG. 1, is that the projected image of LED 10 is in focus at only one distance, resulting in a larger than optimum spot at all other distances. A third problem encountered with the use of a rangefinder device of the type shown in FIG. 1 in a photographic camera is the parallax that results between the beam 14 and the optical axis of the viewing or taking lens of the camera.
One solution to the problem of spot size and focus of the image on the image sensor is shown in U.S. Pat. No. 4,248,532 issued Feb. 3, 1981 to Nosler. In the rangefinder device disclosed by Nosler, the sensor array is angularly oriented relative to the optical axis of the imaging lens 16 (as shown in phantom in FIG. 1 of the present specification) to insure that the spot will remain in sharp focus on the sensor throughout the useful range of object distances. The light beam disclosed by Nosler is generated by a laser, thereby producing an optimum size spot throughout the useful range of distances.
The solutions proposed by Nosler do not solve the problem of parallax between the rangefinder axis and the optical axis of the viewing or taking lens of a photographic camera.